The present invention relates generally to a method and apparatus for detecting acoustic energy and relates more specifically to microstructures that are tunable with energy other than acoustic energy.
Many acoustic sensors include sound-responsive elements that resonate when they are exposed to acoustic energy. Such sound-responsive elements are typically mechanical structures that include any one of the following: a stretched membrane, a clamped diaphragm, a magnetic diaphragm held in place by magnetic forces, conical diaphragms, circular pistons, and corrugated-ribbon conductors. To transform the acoustic energy detected by the aforementioned sound-responsive elements, transducers are coupled to the sound-responsive elements. Typical transducers include the following: loose-contact transducers; moving-iron transducers; electrostatic transducers; piezoelectric transducers; and moving coil transducers.
The transducers and their corresponding sound-responsive elements have inherent size limitations as well as acoustic frequency response limitations. For example, a carbon microphone employs a diaphragm that vibrates in accordance with impulses of sound and then turns the vibrations into electric energy. The vibrations are converted into electrical energy by a piezoelectric transducing method where the diaphragm compresses the carbon granules which in turn changes the electrical resistance of the carbon granules. The electrical resistance of the carbon granules is detected by a voltage measuring device and can be processed by a correlating device which can associate the measured resistance to an acoustic frequency and acoustic intensity. With the voltage measuring device and piezoelectric tranducing method, the operating range for a carbon microphone is typically between 100 and 5,000 hertz (Hz). The carbon microphone is typically used in the mouth piece or transmitter of a telephone handset.
Carbon microphones have a finite length, width, and height that are dependent upon the range of acoustic frequencies that are intended to be detected by the carbon microphone. Due to this physical size dependency on the range of acoustic frequencies intended to be detected, a carbon microphone is typically bulky. Another drawback of a carbon microphone is that it can generate noise. The noise is typically a result of carbon granules that are packed too loosely. Another disadvantage of the carbon microphone is that if an acoustic frequency of interest lies outside the operating range of the carbon microphone, the actual physical dimensions of the carbon microphone and more specifically the sound-responsive element and transducers of the carbon microphone, must be changed or resized in order to detect such a frequency,
Unlike the transducers of the carbon microphone, a dynamic microphone includes a moving coil transducer to convert the movement of a pole into electrical energy. The pole is attached to a diaphragm and is moved by sound waves striking the diaphragm. A dynamic microphone typically includes many parts: a large base permanent magnet, a coil, a diaphragm held in place by a ring, a washer, and large air spaces disposed within the permanent magnet. The dynamic microphone typically has an acoustic frequency response of 30 Hz to 18 kHz. While the dynamic microphone may a large acoustic frequent response range relative to the carbon microphone, the number and size of the dynamic microphone""s operating components render it impractical for use in acoustic arrays and other array like applications. Similar to the carbon microphone, the dynamic microphone is typically designed for a set or fixed acoustic frequency range and therefore, requires substantial mechanical retrofitting or resizing to alter its acoustic frequency range response.
In addition to the size and frequency response limitations of both the dynamic microphone and carbon microphone, these microphones can cause mutual interference in large array applications. For example, if several carbon or dynamic microphones having the same frequency range are placed adjacent to one another, neighboring microphones may interfere with one another due to the sound-responsive elements of each of these microphones being tuned to the same acoustic frequency. Vibrations of one sound responsive element of a microphone may generate acoustic energy or noise relative to a neighboring microphone having a similar size sound-responsive element.
Thus there is a need for an acoustic sensor which avoids the drawbacks of the size and sensitivity of conventional acoustical sensors. There is further need for an acoustic sensor having reduced number of sound-responsive elements. Another need exists for an acoustic sensor that can reduce mutual interference between acoustic sensors placed in an array.
The present invention overcomes the problems associated with conventional acoustic detectors which have inherent limitations based on their size and sensitivity. The acoustic sensor of the present invention detects acoustic energy by entirely different sound-responsive elements and transducers. Consequently the size and frequency response limitations associated with conventional acoustic sensors are substantially eliminated by the present invention. Further, the acoustic sensor of the present invention has a reduced number of moving parts for the sound responsive element and also has the capability of substantially reducing or eliminating mutual interference between neighboring sound responsive elements in acoustic sensor array applications. The acoustic sensor of the present invention has increased sensitivity over a wide range of acoustic frequencies and has dimensions that are orders of magnitude smaller than conventional acoustic sensors. Additionally, the frequency range of the acoustic sensor of the present invention can be modified without changing the actual size of the sound responsive element.
Stated more specifically, the present invention relates generally to an acoustic sensor using microstructures tunable through energy being applied to the microstructure to create stress within the microstructure. The acoustic sensor of the present invention comprises a microstructure tuned to a predetermined acoustic frequency. The sensor also includes a device for detecting movement of the microstructure caused by acoustic energy. The sensor further includes a display device operatively linked to the detecting device. When acoustic energy strikes the sensor, acoustic energy having the predetermined frequency moves the microstructure and the movement is detected by the movement or deflection detecting device.
In addition to the movement detecting device, the acoustic sensor of the present invention can include a device for associating movement of the microstructure with a predetermined acoustic energy level. With such a correlating device, the sensor can output an intensity level of the acoustic energy being detected.
To determine the acoustic intensity energy level and presence of an acoustic frequency, the acoustic sensor detects acoustic energy with a microstructure that has a predetermined acoustic resonant length. The microstructure of the present invention can be any one of the various structures that make-up microelectromechanical systems (MEMS). Specifically, the microstructure of the present invention can be a microbar/microcantilever, a microbridge, or a microplate.
While the aforementioned microstructures of the present invention can be tuned to a predetermined acoustic resonant length, the present invention can also include a device for tuning a microstructure above or below the predetermined acoustic resonant length of a respective microstructure. This tuning device produces energy other than acoustic energy that is focused on the microstructure such that the natural acoustic resonant frequency that is dependent on the length of microstructure is altered. The energy that alters the natural acoustic resonant frequency of the microstructure can be either light energy, thermal energy, energy derived from an electric field, or mechanical energy.
Similar to the various energy forms available to the tuning device, the movement detecting device of the present invention can measure movement of the microstructure through a variety of measuring techniques. In one embodiment, the detecting device can include a laser device and light detector. In another embodiment, the movement detecting device of the present invention can include a capacitive sensing device. And further, the movement detecting device can include a voltage measuring device when the microstructure is made from piezoresistive materials. Alternatively, the detecting device can include a mechanism that monitors a change of resonance of the microstructure itself.
In another aspect of the present invention, the acoustic sensor includes a microstructure array tuned for a predetermined acoustic frequency range. The detecting device of this aspect of the present invention detects movements of individual microstructures in the microstructure array. The acoustic sensor further includes a correlating device that associates movement of individual microstructures with corresponding acoustic resonant frequencies. The acoustic sensor also includes a display device operatively linked to the movement detecting device.
According to this aspect of the invention, the detecting device monitors a selected microstructure at one time in the microstructure array. In order to monitor a selected microstructure, the acoustic sensor can further include a detuning device that detunes or tunes neighboring microstructures away from a natural acoustic resonant frequency of a selected microstructure in the microstructure array. The detuning device exposes the neighboring microstructures with energy other than acoustic energy. This energy can either be light energy, thermal energy, energy derived from an electric field, or mechanical energy.
The microstructure array can also be a two dimensional microstructure array or a three dimensional microstructure array. With a three dimensional microstructure array, the correlating device of the acoustic sensor can determine a direction of a source of the acoustic energy.
In another aspect for the present invention, the acoustic sensor can identify a target based upon a target""s acoustic signature. More specifically, the target sensor includes a microstructure array tuned for a predetermined acoustic frequency range and a detecting device which detects movement of individual microstructures in the microstructure array. The sensor of this aspect of the present invention includes a correlating device that associates movement of the individual microstructures with corresponding acoustic frequencies. The correlating device further associates the detected acoustic frequencies with stored acoustic frequencies of targets in a data base. The sensor further includes a display device that displays detected acoustic frequencies and target identification received from the correlating device.
In yet another aspect of the present invention, a method for detecting acoustic energy includes tuning a microstructure to a predetermined acoustic frequency and exposing the microstructure to acoustic energy. The acoustic sensor then detects movement of the microstructure and then activates a display device to indicate that a predetermined acoustic frequency has been detected.
According to another aspect of the present invention, a method for detecting acoustic energy includes the steps of tuning a microstructure array for a predetermined acoustic frequency range and exposing the microstructure array to acoustic energy. The acoustic sensor detects movement of individual microstructures in the microstructure array and associates the movement of the individual microstructures with corresponding acoustic resonant frequencies. After some calculations or signal processing or both, the sensor displays detected acoustic frequencies on a display device.
In an additional aspect of the present invention, a method for identifying a target employs a microstructure array tuned for a predetermined acoustic frequency range. The microstructure array is then exposed to acoustic energy generated by a target. The target sensor of the method for identifying a target then detects movement of individual microstructures in the microstructure array and associates the movement of the individual microstructures with corresponding acoustic resonant frequencies. The target sensor then associates detected acoustic frequencies with stored acoustic frequencies which targets in a database. The target sensor then displays detected acoustic frequencies in identification of the target on a display device.
Thus it is an object of the present invention to provide an improved acoustic sensor.
It is another object of the present invention to tune an acoustic sensor having a microstructure by stressing the microstructure with energy other than acoustic energy with out altering or changing the physical dimensions of the microstructure. A further object of the present invention is to provide an array of microstructures that detect acoustic energy where performance of a selected microstructure is not affected by neighboring microstructures.
Other objects, features, and advantages of the present invention will become apparent upon reading the following specification, when taken in conjunction with the drawings and dependent claims.